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(3) You must attribute this AAM in the following format: Creative Commons BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/deed.en), doi:10.1016/j.lithos.2009.12.013 1 2 Carboniferous high-pressure metamorphism of Ordovician protoliths 3 in the Argentera Massif (Italy), Southern European Variscan belt 4 5 Daniela Rubatto 6 Research School of Earth Sciences, The Australian National University, Canberra 7 0200, Australia 8 9 Simona Ferrando, Roberto Compagnoni 10 Dipartimento di Scienze Mineralogiche e Petrologiche, Universita’ degli Studi di 11 Torino, Via Valperga Caluso 35, Torino 10125, Italy 12 13 Bruno Lombardo 14 C.N.R., Istituto di Geoscienze e Georisorse, Via Valperga Caluso 35, Torino 10125, 15 Italy 16 17 [email protected] 18 Tel. ++61 (0)2 6125 5157 19 Fax ++61 (0)2 6125 0941 20 21 22 23 1 24 ABSTRACT 25 The age of high-pressure metamorphism is crucial to identify a suitable tectonic 26 model for the vast Variscan orogeny. Banded HP granulites from the Gesso-Stura 27 Terrain in the Argentera Massif, Italy, have been recently described (Ferrando et al., 28 2008) as relict of high-pressure metamorphism in the western part of the Variscan 29 orogen. Bulk rock chemistry of representative lithologies reveals intermediate silica 30 contents and calc-alkaline affinity of the various cumulate layers. Enrichment in 31 incompatible elements denotes a significant crustal component in line with intrusion 32 during Ordovician rifting. Magmatic zircon cores from a Pl-rich layer yield scattered 33 ages indicating a minimum protolith age of 486±7 Ma. Carboniferous zircons 34 (340.7±4.2 and 336.3±4.1 Ma) are found in a Pl-rich and a Pl-poor layer, 35 respectively. Their zoning, chemical composition (low Th/U, flat HREE pattern and Ti- 36 in-zircon temperature) and deformation indicate that they formed during the high- 37 pressure event before decompression and mylonitisation. The proposed age for high- 38 pressure metamorphism in the Argentera Massif proves that subduction preceded 39 anatexis by less than 20 Ma. The new data allow a first-order comparison with the 40 Bohemian Massif, which is located at the eastern termination of the Variscan orogen. 41 Similarities in evolution at either end of the orogen support a Himalayan-type 42 tectonic model for the entire European Variscides. 43 Keywords HP granulites, U-Pb geochronology, zircon, Variscan belt. 44 45 2 46 1. Introduction 47 The Variscan orogeny (~380-300 Ma) is the geological event most largely 48 represented in the basement of the European continent. It was assembled between 49 Ordovician and Carboniferous from the larger collision of Gondwana with the 50 northern plate of Laurentia-Baltica, which involved the microplates of Avalonia and 51 Armorica (Matte, 2001). Variscan units extend from southern Spain (the Ibero- 52 Armorican termination) to Poland (the Bohemian Massif). Large remnants of Variscan 53 basement are preserved in the southern Variscides, within the Alpine chain, where 54 they are located in external positions. In the Western and Central Alps, such 55 remnants are identified as External Crystalline Massifs, which record the general 56 evolution common to all Pangean Europe (von Raumer et al., 2009). 57 A series of tectonic models have been proposed for the assembly of this vast 58 orogen. Early models favour Himalayan-style collision with subduction of a small 59 ocean rapidly followed by intense continent-continent collision leading to Barrovian 60 metamorphism and extensive crustal anatexis in the Late Carboniferous (summary in 61 O'Brien, 2000). More recently, Andean-style tectonics has been proposed, at least for 62 the eastern termination of Variscan Europe (Bohemian Massif). The Andean model 63 prefers a long lasting subduction process with development of blueschist terranes, 64 extensive arc magmatism in the upper plate and formation of back-arc basins 65 (Schulmann et al., 2009). 66 One crucial piece of information that is necessary in order to better define a 67 suitable geodynamic model for the Variscan orogen is the absolute and relative ages 68 of subduction (as seen in relicts of eclogites) versus the onset of regional anatexis. 69 Whereas the latter event is reasonably well constrained across the western European 70 Variscan basement at around 320-310 Ma (e.g. Demoux et al., 2008; Rubatto et al., 71 2001), the scarcity of eclogite facies rocks and their poor preservation have 3 72 hampered robust dating of Variscan high-pressure (HP) assemblages. Some 73 constraints exist for the eastern part of the orogen (Bohemian Massif, Kröner et al., 74 2000; Schulmann et al., 2005), but ages of HP assemblages are lacking in the 75 western part. This contribution presents the first geochronological constraints 76 (SHRIMP U-Pb dating of zircon) on HP assemblages recently described in the 77 Argentera Massif. This is a crucial record for the External Crystalline Massifs and for 78 most of the western portion of the European Variscan orogen. 79 80 2. Geological background and previous geochronology 81 The Argentera Massif is located in NW Italy, on the border with France. It is the 82 southernmost of the External Crystalline Massifs, which are a series of large crustal 83 bodies aligned on the external part of the western and central Alpine chain (Fig. 1a). 84 They are generally composed of a complex Variscan basement intruded by Permian 85 granitoids. Alpine overprint in these Massifs is weak and commonly limited to shear 86 zones. The exhumation of the External Crystalline Massifs from below the Alpine 87 sediments initiated in the Miocene (e.g. Bigot-Cormier et al., 2006), at the end of the 88 Alpine orogeny. 89 The Argentera Massif is largely composed of Variscan migmatites with abundant 90 relicts of pre-anatectic rock types. At the centre of the Massif, a post-Variscan 91 granite (the Central Granite, Fig. 1b) cuts across the foliation. The Massif is 92 subdivided into two major complexes on the basis of different lithological 93 associations: the Gesso-Stura Terrain in the NE, and the Tinée Terrain in the SW. A 94 large shear zone, the Ferriere-Mollières Line, separates the two Terrains. The studied 95 Frisson Lakes area is located at the eastern tip of the Gesso-Stura Terrain, which is 4 96 mainly composed of migmatitic ortho- and para-gneisses, with various intrusive 97 bodies from mafic (Bousset-Valmasque Complex) to granitic in composition. 98 A Late- to Mid-Carboniferous age (≤323± 12 Ma) of migmatisation in the 99 Argentera Massif has been proposed on the basis of a zircon lower intercept age 100 obtained for the Meris eclogite (Rubatto et al., 2001), the only relict of fresh eclogite 101 so far dated. Migmatisation in the Gesso-Stura Terrain must have occurred after the 102 intrusion of monzonites (332±3 Ma, Rubatto et al., 2001), which show signs of 103 partial melting, and before the intrusion of the Central Granite (~285-293 Ma, 104 Ferrara and Malaroda, 1969). For the Tinée Terrain, an earlier age (~350 Ma) of 105 metamorphism has been proposed on the basis of scattering Ar-Ar ages of muscovite 106 from gneisses (Monié and Maluski, 1983). Alpine low-grade overprint along shear 107 zones occurred in or before the Early Miocene (Corsini et al., 2004). 108 Additional constraints on Variscan migmatisation come from the nearby massif of 109 Tanneron (Fig. 1a), SE France, where migmatitic rocks contain monazites dated 110 between ~317 and 309 Ma (Demoux et al., 2008). In contrast, in Variscan Corsica, a 111 few zircon rims in a migmatitic paragneiss yielded an age of 338±4 Ma (Giacomini et 112 al., 2008), interpreted as dating “incipient migmatisation”. 113 Geochronology of pre-anatectic events in the Argentera Massif is scarce and 114 mainly limited to magmatic activity. U-Pb zircon dating has returned the age of Late 115 Ordovician bimodal magmatism (~440 and 460 Ma) and of Carboniferous monzonites 116 (Rubatto et al., 2001). Previous attempts to date metamorphic rocks either returned 117 contrasting results (Paquette et al., 1989) or failed to date metamorphism (Rubatto 118 et al., 2001). 119 120 3. Analytical methods 5 121 Whole-rock major- and trace-element compositions were analysed at the Chemex 122 Laboratories (Canada) using ICP-AES (major elements) and ICP-MS (trace elements). 123 The precision for the analyses is better than 1% for major elements and better than 124 5% for trace elements. Zircons were prepared as mineral separates mounted in 125 epoxy and polished down to expose the grain centres. Cathodoluminescence (CL) 126 imaging was carried out at the Electron Microscope Unit, The Australian National 127 University with a HITACHI S2250-N scanning electron microscope working at 15 kV, 128 ~60 µA and ~20 mm working distance. 129 U-Pb analyses were performed using a sensitive, high-resolution ion microprobe 130 (SHRIMP II) at the Research School of Earth Sciences.